This grant is based on several central postulates: (1) diet and nutritional status are among the most important, modifiable determinants of global human health; (2) the nutritional value of food is not an absolute entity, but rather a relative term influenced in part by a person's gut microbiota and microbiome; (3) there is a dynamic interrelationship between diet and the structure and operations of our gut microbiota/microbiomes; (4) the gut microbiota and its microbiome should be considered as factors in our energy balance equation and thus interpersonal variations in the structure and operations of this community may affect risk for development of obesity (or malnutrition); (5) the gut microbiota, and the host genes it manipulates to modulate nutrient processing/energy balance are attractive targets for treatment or prevention of obesity, and for a more informed, more personalized era of human nutrition. DK70977 has 2 specific aims. Aim 1 - Use a randomized factorial diet oscillation study design, elaborated based on successive rounds of data modeling, where diets with systematically altered carbohydrate, protein and fat content are sequentially presented, in varied order, to individual gnotobiotic mice harboring a defined consortia of sequenced human gut microbes, with and without hydrogen-consuming methanogens, acetogens and sulfate reducing bacteria, and the effects on microbial community composition, gene expression, and metabolism are determined using a variety of methods centered around the massively parallel Illumina GA-II DNA sequencer [multiplex shotgun sequencing of random genomic fragments generated from fecal DNA to quantify community membership; sequencing of cDNA, generated from rRNA-depleted community RNA preparations (microbial RNASeq)], and RNASeq-informed metabolite quantification. Aim 2 - Use animals from Aim 1 to study the impact of diet, and the representation and metabolic activities of hydrogen consumers, on energy extraction from food, host energy balance (using serial physiological profiling methods within gnotobiotic isolators), and intestinal expression of G-protein coupled receptors plus genes affected by products emanating from sulfate reducing bacteria. The proposed proof-of-principle/proof-of-mechanism experiments should help efforts to develop new ways for assessing nutritional status, for making nutritional recommendations, and for determining whether targeted manipulation of the representation or metabolic activities of hydrogen-consuming gut microbes would provide a safe and effective means for controlling energy and nutrient flux from diet to host.